Re: [PATCH] x86 Boot NUMA kernels on non-NUMA hardware with DISCONTIG memory model

[Andy Whitcroft <apw@shadowen.org>]

Andi Kleen wrote:
>> This reserved portion of the KVA must be PMD aligned. 
> 
> Why do they need to be PMD aligned?  

That comes from the fact that the KVA in x86 has traditionally been
mapped with huge pages where at all possible, for performance reasons.
The purpose of the remap itself always has been performance based, we
are remapping node-local memory into KVA to hold the memmap in part to
exploit locality of process to its memory, and to in part to distribute
the load on the NUMA memory infrastructure by "striping" the storage.
As a result it makes sense to map these remapped areas with huge pages
also.  As is evidenced by the fact this bug is only coming to light now,
it is somewhat rare for the end of a NODE to be miss-aligned below the
huge page level (2/4MB).

-apw